ILLUMINATION FOR OPAQUE OBJECTS. 



177 



FIG. 55. 



Arrangement of Microscope for Opaque Objects. 



For all ordinary purposes, a piano or double-convex lens, of 

 about 1J inch diameter, and 2 inches focus, either mounted upon 

 a separate stand (as in Fig. 45), or so attached by a jointed 

 support to the Microscope itself as to admit of being placed in 

 any required position, 

 will answer extremely 

 well as a Condenser. 

 If Daylight be em- 

 ployed, the micro- 

 scope should be so 

 placed that the strong- 

 est light may fall 

 obliquely upon the 

 stage, and preferably 

 from the left hand 

 side ; there will then 

 be no difficulty in so 

 disposing this con- 

 denser, as to afford 

 an illumination suffi- 

 cient for almost any 

 kind of object, pro- 

 vided the quality of 

 the light itself be good. Direct sunlight cannot be here em- 

 ployed, without the production of an injurious glare, and the 

 risk of burning the .object; but the sunlight reflected from a 

 bright cloud is the best light possible. The condenser should 

 always be placed at right angles to the direction of the illumi- 

 nating rays, and at a distance from the object which will be de- 

 termined by the size of the surface to be illuminated and by the 

 kind of light required. If the magnifying power employed be 

 high, and the field of view be consequently limited, it will be 

 desirable so to adjust the lens, as to bring the cone of rays to a 

 point upon the part of the object under examination ; and this 

 adjustment can only be rightly made whilst the object is kept in 

 view under the microscope, the condenser being moved in va- 

 rious modes, until that position has been found for it in which 

 it gives the best light. If, on the other hand, the power be low, 

 and it be desired to spread the light equably over a large field, 

 the condenser should be placed either within or beyond its focal 

 distance ; and here, too, the best position will be ascertained by 

 trial. It will often be desirable, also, to vary both the obliquity 

 of the light, and the direction in which it falls upon the object ; 

 the aspect of which is greatly affected by the manner in which 

 the shadows are projected upon its surface, and in which the 

 lights are reflected from the various points of it. There are 

 many objects, indeed, distinguished by their striking appearance 

 when the light falls upon them on one side, which are entirely 

 destitute both of brilliancy of color and of sharpness of outline, 



12 



